Dual-polarized, sideband-separating, balanced receivers are well known and have been used extensively at microwave and millimeter-wave frequencies. However, it is presently believed that receivers based on prior art designs may not operate well at frequencies at or above 1 THz. Only recently, sideband-separating balanced receivers have been designed for frequencies beyond W-band (75-110 GHz). For example, receiver architectures have been proposed for frequencies up to 900 GHz for the Atacama Large Millimeter/Submillimeter Array (ALMA).
Dual-polarized receivers detect both polarizations of incoming radiation. When both polarizations are received simultaneously, there is a √{square root over (2)} improvement in signal-to-noise ratio (SNR), or a factor of two reduction in observing time. In some prior art applications, dual polarization operation may be achieved by using a wire-grid polarizer to split the telescope beam into two polarizations. The output of a local oscillator (LO) may be injected using a beamsplitter, either after the polarizer, in which case two beamsplitters are used; or before the polarizer, where a single, correctly oriented beamsplitter is used. Either approach leads to fairly complicated optical designs, especially for receivers with multiple bands or multiple pixels.
Most submillimeter-wave receivers in radio astronomy currently use double-sideband (DSB) mixers to down convert an RF (Radio Frequency) signal to an intermediate frequency. DSB mixers are useful for continuum observations, where the signals from both sidebands are equally important. However, for spectral line observations, the presence of the down-converted signals from the unwanted image band may degrade receiver sensitivity and calibration certainty. Therefore, sideband-separating receivers with good image rejection capability are desirable for high resolution spectral line observations. Moreover, sideband-separating receivers with good image rejection may mitigate confusion from spectra in the image sideband, and may mitigate calibration uncertainty from sideband imbalance.
Balanced mixers use two or more detector elements in a balanced configuration to help suppress local oscillator amplitude modulation (AM) noise, help provide better power handling capabilities than unbalanced mixers, and help reject certain spurious responses and spurious signals. By simplifying LO injection and eliminating the need for diplexers, balanced mixers are desirable components for scaling receivers to multi-pixel arrays.